The present invention relates to a method and/or architecture for a voltage controlled oscillator generally and, more particularly, to a method and/or architecture for an automatic gain control circuit for use with a voltage controlled oscillator.
A voltage controlled oscillator (VCO) is commonly employed in a variety of applications, including communications, and timing circuitry. In particular, a VCO is commonly used in phase locked loop (PLL) control systems. Functionally, a VCO may be viewed as a circuit that seeks to transform an input control voltage signal into an output frequency signal in a substantially linear fashion. The frequency gain of a VCO typically defines the relationship between changes in the output frequency relative to predetermined changes in the input control voltage signal (i.e., delta frequency/delta voltage).
Referring to FIG. 1, a graph illustrating a linear frequency gain is shown. The linear relationship between the frequency of the signal FOUT and the magnitude of the voltage signal VIN may be expressed using the following equations:                     ⅆ        f                    ⅆ        v              =    m                      ∫        f0                  f          n                    ⁢              ⅆ        f              =          m      ⁢              xe2x80x83            ⁢                        ∫          v0                      v            n                          ⁢                  ⅆ          v                    
fn=m(vnxe2x88x92v0)+f0
where m is the constant VCO gain, f0 and v0 are the initial values of the frequency of the signal FOUT, and fn and vn are the output frequency and the input voltage.
Referring to FIG. 2, a block diagram of a linear VCO 10 is shown. The VCO 10 includes an output stage 12 and an input stage 14. The output stage 12 transforms a control current Ic into an output signal FOUT having a predetermined frequency based on the magnitude of the control current Ic. The input stage 14 converts an input voltage VIN into the control current Ic. For example, the output stage 12 can include a plurality of differential current switches connected in series with an output of a last current switch connected to an input of a first current switch (i.e., a so-called ring oscillator). The control current Ic is mirrored into the plurality of current switches controlling the output frequency (i.e., controlling the biasing current), or, alternatively, the magnitude of the control current is used to control a load associated with the plurality of current switches thereby varying the output frequency of the VCO.
The input stage 14 includes a transistor 16 and a transistor 18. The signal VIN is presented to a gate of the transistor 16. A resistor 20 couples a source of the transistor 16 to a voltage supply ground VSS. A drain of the transistor 16 is connected to a drain and a gate of the transistor 18. A source of the transistor 18 is connected to a supply voltage VCC. A voltage across the resistor 20 varies in response to the signal VIN generating the control current Ic. The control current Ic is mirrored from the transistor 18 to the output stage 12. Referring to FIG. 3, a schematic diagram illustrating a transistor implementation of the circuit 10 is shown.
Referring to FIG. 4, a graph illustrating the frequency gain for the circuit 10 is shown. The input stage 14 of a conventional VCO has a linear responsexe2x80x94ostensibly in order to effect a linear VCO input/output response. The resistor 20 provides a constant VCO gain that can be used with a phase lock loop (PLL) for a stable control system that is not sensitive to change.
In certain applications, the linear gain of the VCO 10 may be a disadvantage. For example, a spread spectrum application can require a VCO with a nonlinear gain (i.e., a low gain at low frequencies and a high gain at high frequencies). A description of a spread spectrum application may be found in co-pending application U.S. Ser. No. 09/618,622, filed concurrently, which is hereby incorporated by reference in its entirety. Since the gain of the circuit 10 is controlled by the resistor 20, the circuit 10 cannot achieve a nonlinear gain.
The present invention concerns an apparatus comprising a circuit configured to generate an output signal having a frequency that varies in response to a voltage signal, wherein said circuit has an automatically controlled nonlinear frequency gain.
The objects, features and advantages of the present invention include providing a method and/or architecture for automatic gain control in a voltage controlled oscillator (VCO) that may (i) provide an adaptive spread spectrum device, and/or (ii) provide a wider range of operating frequency than a conventional VCO.